Fair Value Measurements - Recurring Basis (Details) - Recurring - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	$ 4,121	$ 4,499
Fair value measurements using quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	4,107	4,499
Fair value measurements using significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	14	0
Fair value measurements using significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0	0
Cash and cash equivalents | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	3,925	4,405
Cash and cash equivalents | Foreign government treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	32	39
Cash and cash equivalents | Fair value measurements using quoted prices in active markets for identical assets (Level 1) | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	3,925	4,405
Cash and cash equivalents | Fair value measurements using quoted prices in active markets for identical assets (Level 1) | Foreign government treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	32	39
Cash and cash equivalents | Fair value measurements using significant other observable inputs (Level 2) | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0	0
Cash and cash equivalents | Fair value measurements using significant other observable inputs (Level 2) | Foreign government treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0	0
Cash and cash equivalents | Fair value measurements using significant unobservable inputs (Level 3) | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0	0
Cash and cash equivalents | Fair value measurements using significant unobservable inputs (Level 3) | Foreign government treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0	0
Other current assets | U.S. treasuries and government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	150	55
Other current assets | Foreign currency forward contracts | Designated as Hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	13
Other current assets | Foreign currency forward contracts | Not Designated as Hedging Instrument
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	1
Other current assets | Fair value measurements using quoted prices in active markets for identical assets (Level 1) | U.S. treasuries and government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	150	55
Other current assets | Fair value measurements using quoted prices in active markets for identical assets (Level 1) | Foreign currency forward contracts | Designated as Hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0
Other current assets | Fair value measurements using quoted prices in active markets for identical assets (Level 1) | Foreign currency forward contracts | Not Designated as Hedging Instrument
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0
Other current assets | Fair value measurements using significant other observable inputs (Level 2) | U.S. treasuries and government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0	0
Other current assets | Fair value measurements using significant other observable inputs (Level 2) | Foreign currency forward contracts | Designated as Hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	13
Other current assets | Fair value measurements using significant other observable inputs (Level 2) | Foreign currency forward contracts | Not Designated as Hedging Instrument
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	1
Other current assets | Fair value measurements using significant unobservable inputs (Level 3) | U.S. treasuries and government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0	0
Other current assets | Fair value measurements using significant unobservable inputs (Level 3) | Foreign currency forward contracts | Designated as Hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0
Other current assets | Fair value measurements using significant unobservable inputs (Level 3) | Foreign currency forward contracts | Not Designated as Hedging Instrument
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0
Accrued expenses and other liabilities | Foreign currency forward contracts | Designated as Hedges
Liabilities measured at fair value on a recurring basis, gain loss included in earnings
Total liabilities at fair value	(1)	(5)
Accrued expenses and other liabilities | Fair value measurements using quoted prices in active markets for identical assets (Level 1) | Foreign currency forward contracts | Designated as Hedges
Liabilities measured at fair value on a recurring basis, gain loss included in earnings
Total liabilities at fair value	0	0
Accrued expenses and other liabilities | Fair value measurements using significant other observable inputs (Level 2) | Foreign currency forward contracts | Designated as Hedges
Liabilities measured at fair value on a recurring basis, gain loss included in earnings
Total liabilities at fair value	(1)	(5)
Accrued expenses and other liabilities | Fair value measurements using significant unobservable inputs (Level 3) | Foreign currency forward contracts | Designated as Hedges
Liabilities measured at fair value on a recurring basis, gain loss included in earnings
Total liabilities at fair value	$ 0	$ 0